The invention relates to a lens arrangement for geometrically separating the light fluxes of imaging systems into light ray bundles of different positions and directions of extension. The arrangement includes an object imaging optical system defining at least two planes which are complementary to one another, preferably in infinity. Planes are complementary to each other if in a given area of one plane any point in this area is connected to each point of a given area in the other plane by geometrical rays.
Imaging systems which utilize emitted and returning light rays, e.g. concave mirror systems, and systems of which the image plane and the object plane are not accessible present difficulties with regard to their light paths when utilized in praxi. When light is introduced into such systems across the whole free diameter of the systems by means of partially light transmitting mirrors--as for example described in the German Pat. No. 1 145 807--and thereafter the emitted light rays are separated from the returning light rays by means of the same partially transmitting mirror difficulties are encountered which are caused by reflexes, scattered light, astigmatism of inclined parallel glass plates etc., which cause a deterioration of contrast and image quality. To a certain extent these difficulties may be overcome by geometrically separating the emitted from the returning light rays and allocating the separated light ray bundles to different portions of the object field or the image field which may be imaged one upon the other. This method is, for example, used with autocollimating telescopes.
In other imaging systems light rays which extend in the same direction but pass through different portions of the object field or image field respectively transmit the same portions of the Fourier plane in infinity related to such object field or image field. Thereby even more severe difficulties are caused if light ray bundles are to be separated which travel through the object field or image field respectively and extend in different directions because their light fluxes represent different parameters of the measurement and are, therefore, modulated with different frequencies, phases, and amplitudes.
Such systems are described, for example, in "Philips Technische Rundschau" (Philips Technical Survey), 1969/1970, page 153 etc. and page 158, and, further by the applicant in the Conference Report of the "Conference on Moire Fringe Technology" held June 19/20, 1972 at Birnihill Institute, National Engineering Laboratory, East Kilbride Glasgow, Paper 8a, FIG. 25. These descriptions relate to position measuring systems for example in step repeaters measuring in one or two coordinates which by means of concave mirror lens systems are adapted to produce large flat object fields or image fields respectively having good chromatic correction. When phase gratings are imaged one upon another, which produces a high light efficiency and a good signal characteristic, a modulation of light fluxes is generated in such step repeaters by the movement of the grating and the grating image relative to one another which light fluxes are modulated with different phase relation according to the interference of different orders of diffraction. Consequently, the light rays of different areas of the solid angle of diffracted beams must be allocated to separated photoelectric receivers in order to obtain the desired signal components of rotating electrical fields.
However, this method has the disadvantage that the size of the illumination aperture causes the cross section of the used light bundles to be unsharp or fuzzy. In order to separate the light ray bundle which illuminates the object field clearly from the adjacent light ray bundle which illuminates the image field both light ray bundles must have a distance from one another which is determined by the illumination aperture. This is achieved by an inclined position of the light ray bundles relative to one another. This inclination, however, requires more space and renders the device rather voluminous. The unsharpness caused by object illumination aperture further causes a considerable loss of light efficiency in the whole system.
It is therefore an object of the present invention to provide an imaging arrangement by which the light flux may be separated both according to areas of the object field and of the image field and according to the direction of the light rays in these field areas when the imaging planes or the Fourier plane are directly inaccessible. And it is a further object to provide such an arrangement which requires less space and has an improved light transmission efficiency.